#!/usr/bin/node
const Readable=require('stream').Readable;
let c='a'.charCodeAt(0);
class MyReadable extends Readable{
  constructor(){
    super();  
  }
  _read(){
    rs.push(String.fromCharCode(c++));
    if(c>'z'.charCodeAt(0)) rs.push(null);
  }

}
let rs=new MyReadable();
rs.pipe(process.stdout);
